1. Field of the Invention
The present invention relates to a glass circuit substrate that is obtained by forming a metal layer on a glass substrate and forming wiring and electrodes therein and a fabrication method thereof. More particularly, the present invention relates to a glass circuit substrate having the wiring and electrodes formed with high adhesion by wet plating while retaining smoothness of the surface of glass substrate, and a fabrication method thereof.
2. Related Background Art
The following is one of the conventionally known methods for metallizing glass, which is an insulating substrate, by wet plating. Specifically, the surface of glass is first degreased to be cleaned, then the surface is roughened by etching. Therafter, it is dipped in Sn--Pd colloid solution to make the roughened surface capture colloidal particles, and then a tin overcoat of tin-palladium colloid is removed by an acid, thereby obtaining Pd metal nuclei for plating.
The substrate thus processed is dipped in an electroless nickel--phosphorus (hereinafter referred to as Ni--P) plating solution, whereby an electroconductive metal layer of Ni--P plating film is formed on the surface of substrate with the above Pd metal nuclei acting as catalyst nuclei.
A prior art example is a research report presented by Honma et al. (Hyomen Gijutsu (Surface Technology) vol. 44, No. 10, 1993) to describe the method for roughening glass with 10% hydrofluoric acid, then providing it with the Pd nuclei, and thereafter forming the electroless Ni--P plating.
For forming a metal wiring pattern on the insulating substrate by plating, normally, the above Ni--P plating film is deposited in about 1 .mu.m on the surface of the insulating substrate and thereafter a low-resistance metal layer of Cu plating or the like is deposited thereon in the thickness of several pm to several ten .mu.m.
In this case, the Ni--P plating film plays a role like an undercoat layer for depositing the low-resistance metal plating film on the surface of insulating substrate.
In addition to the metallizing method described above, there is another method for coating the surface of a substrate with an amino silane coupling agent having one end at a functional group with affinity to the surface of insulating substrate and at the other end an amino group, or a like agent, in contact with palladium chloride solution so as to make the amino groups capture Pd ions, and then reducing the Pd ions to metal Pd with a reducing agent such as sodium hypophosphite. A metal layer can be formed on the catalyst nuclei formed in this way, by use of the electroless nickel plating or the like.
Another prior art example is a method for forming an electroconductive layer on an insulating member, in which a semiconductor layer of either ZnO or WO.sub.3 is formed on the insulating member, a film of Pd, Pt, Au, Ag, or the like is then deposited thereon, and thereafter the electroconductive layer of Cu or the like is placed thereon, which was disclosed in applications filed by Fujishima et al. (Japanese Laid-open Patent Application No. 6-61619 and Japanese Laid-open Patent Application No. 4-17211).
For forming a desired metal pattern on these insulating substrates, a known method is the subtractive method for first forming a necessary metal layer over the entire surface of the substrate by plating, then protecting the metal layer in necessary portions by a photoresist process or the like, and removing non-protected portions with an etchant.
The plating film formed on the conventional glass substrates, however, had the following problems.
For achieving plating of the metal layer on the glass surface with high adhesion, the conventional technology needed to roughen the surface with hydrofluoric acid. Then the roughened substrate is dipped in a sensitizing solution containing approximately 0.1 g/l of stannous chloride (SnCl.sub.2) to undergo a sensitizing process, thereby making the roughened surface capture Sn.sup.2+ ions.
This substrate is dipped in an activation solution (containing about 0.1 g/l of palladium chloride) to substitute the Sn.sup.2+ ions captured for Pd.sup.2+, ions, thereby forming Pd catalyst nuclei for electroless plating. In addition to this pretreatment method, another known method is the alkali catalyst method for making an organic complex compound of Pd adsorb to the surface of glass substrate at pH 9 to 13.
Then an electroless Ni--P plating layer is deposited in about 0.5 (.mu.m) on the catalyst nuclei, a Cu plating film is further deposited up to a necessary thickness (normally about 3 to 20 .mu.m), and Ni and Au plating films are formed as Cu-oxidation inhibitor layers thereon. In the circuit substrate having the above configuration, the adhesive force between the electroless Ni--P plating layer and the glass substrate results from the anchor effect due to intrusion of the plating layer into the roughened surface of glass having complex shapes.
However, if the adhesive force necessary for use for circuit patterns relies only upon the anchor effect, the surface must be roughened before the average surface roughness Ra of glass becomes approximately 0.2 .mu.m. This will degrade such characteristics as transparency and smoothness that the glass substrate originally has. In addition, microcracks more easily occur in the entire surface, and they often cause reductions in the strength of substrate. Further, it becomes hard to ensure the accuracy of fine line pattern necessary for electron devices or the like and there is a problem that a probability of breaking of wire becomes high in patterning of fine lines.
Another problem is that the Ni--P plating film starts being affected by crystallization due to creation of nickel phosphide after the heating temperature becomes over 100.degree. C. and that it is converted to Ni.sub.3 P near 400.degree. C. to cause strain between the Pd catalyst nuclei and the Ni plating layer, thereby degrading adhesion.
There is also a method for providing a coupling agent between the glass substrate and the plating layer in order to avoid the degradation of smoothness of substrate surface, but this method has a problem that annealing is allowed only at temperatures below the decomposition temperature of the coupling agent and that temperatures of the atmosphere used are also limited to those below the decomposition temperature of the coupling agent.
A further method is a one for forming a circuit pattern by printing of a metal paste on glass, but it has the following drawbacks; the metal paste patterned needs to be baked at high temperatures, and the substrate is thus readily deformed upon the baking; only crystalline glass with high heat resistance can be used, which is a cause of increase of cost; it is not easy to achieve flatness of pattern surface by printing.
Here, utilizing the method as disclosed in Japanese Laid-open Patent Application No. 6-61619 and Japanese Laid-open Patent Application No. 4-17211, the plating film can be deposited on the glass substrate with high adhesion, but it necessitates the ZnO or WO.sub.3 film forming process, thus posing a problem of increase in film-forming cost. In addition, if a low-melting-point glass substrate such as blue sheet glass is used, the problem of warp, strain, or the like of the glass substrate will arise, because high temperatures are needed in the film-forming process of ZnO or WO.sub.3.
When the substrate for wiring is made by forming the metal layer on the glass substrate, the following problems are considered.
(1) In the case of printing, after the metal paste printed on the glass substrate is baked, the components other than the metal component in the paste will remain in the wiring, resulting in increasing the resistance. Since the paste is baked at several hundred degrees, the metal component making the wiring will be oxidized in the case of baking in the atmosphere. This is also a cause to increase the wiring resistance. Therefore, the resistance is higher than that of the deposited film mostly made of the metal component.
(2) When the wiring is made by plating on an ITO film, peeling will occur between the glass substrate and the ITO film upon depositing the plating film on the ITO film unless the adhesive force is sufficient between the ITO film and the glass substrate. Especially, in the case of electroless plating, stress is high in the plating film, so that peeling may occur between the glass substrate and the ITO film when the plating film is deposited in 2 to 3 .mu.m.
(3) In the case of direct electroless plating on the glass substrate, when another plating film is further deposited on the electroless plating film, the plating layer may also be peeled off from the glass substrate because of stress of the deposited film, similar to above. As well known, the plating film on the metal undercoat layer does not exhibit so high adhesion to the glass substrate by nature. This means that, in order to form a metal layer with high adhesion on a non-roughened glass substrate such as blue sheet glass, it is important that for formation of the metal layer (the plating film), the substrate and the metal undercoat layer first be connected with each other by strong coupling resistant to the stress of deposit plating film.
On the other hand, there is a technique so called chip on glass (COG) for directly mounting electronic parts such as IC on the glass substrate. Glass circuit substrates applicable to this technique are required to have some high characteristics.
Mass-production methods of COG include the flip chip bonding method (which is a method for bonding a chip to a conductive part of substrate with an electroconductive adhesive) and the wire bonding method.
In this case, wires preliminarily provided with bumps are formed on the glass substrate to which the chip is to be bonded. For this, conventionally known methods for forming the conductive layer with good adhesion on glass or ceramics include a method for forming a film of ITO on the glass substrate by sputtering and building the wiring part up thereon by electroless Ni plating and a method for forming a pattern of a paste containing a metal by screen printing and baking it to form wiring (see Japanese Laid-open Patent Application No. 64-67840). Another example to realize COG is a technique for mounting semiconductor devices for drive on a film of Ni and gold plating on a transparent electrode (see Japanese Laid-open Patent Application No. 2-69720 and Japanese Laid-open Patent Application No. 8-271869).
However, the glass substrates are often used for displays and the like in these years and, particularly, decrease in the wiring resistance is thus indispensable for large-screen arrangement. This is because the large-screen arrangement naturally increases the wiring length so as to raise the need for decreasing the wiring resistance. If the wiring resistance were high, the power applied would be converted to heat and this would further raise the resistance, resulting in failing to supply desired power. There thus remains a problem that the wiring needs to be thick and deep for decreasing the resistance.
An example thereof is as follows. Plasma displays today are often made by forming multilayer coatings of print paste and baking it, and in this case, the wiring after baked is porous and organic materials among the paste components are a cause to obstruct the decrease of resistance. Film formation of ITO by sputtering is normally employed for formation of the conductive layer on the non-roughened glass substrate nowadays. The ITO film, however, has problems that the volume resistivity is of the order of 10E-4 .OMEGA.cm, which is two figures higher than those of the plating films and that the throughput (mass productivity) is relatively low because of use of deposition apparatus.
Thus proposed was a method for forming various micro wires and micro electrodes on the glass substrate by means of plating. In this case, when the glass substrate 41 is roughed as in the conventional method in order to secure adhesion between the glass substrate 41 and the plating film 42 as shown in FIG. 30, the anchor effect can be expected at the interface 43 between them, but the shape of the roughed surface also appears on the plating film 42 as it is. For example, when a thin electrode 44 of a noble metal is made as shown in FIG. 31, there is a problem that a part of the electroless plating film of undercoat is exposed in the electrode surface as indicated by numeral 45 in the drawing, which could degrade reliability of connection with an electronic part and damage characteristics of electrode.
Under such circumstances, it becomes necessary to use a glass substrate in a non-roughed state, for example a glass substrate in a mirror surface state such as float glass or polished glass. Recent trends in development of display seem to be directed toward lightweight, thin, and compact displays, so that compact packaging is also required at present.